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Yup the CPU is the 939 even tho it works on the 940 AM2. Its still AGP or PCIE 1.0. There was lots of claims like you could tap one side of the PCIE connetors to get new PCIE 2.1/3.0 cards to work but never see it done. I would stick with GPU's know to work with the slot.
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I was going to point out that the 4800+ was available for both socket 939 and AM2, but that the 2.5GHz version was only on the newer AM2 socket, though the OP already verified that his motherboard is AM2. I wasn't finding much information on the motherboard from good sources, though what little I've found seems to indicate that motherboard might be able to run some phenom x4 quad-core processors, although it could require a bios update, if it were to work at all. Used Phenom X4s are relatively inexpensive on sites like eBay now, though you might have to do more research to determine whether a particular chip would be compatible with your particular motherboard. And of course, the single-threaded performance on those CPUs is still quite a bit lower than current generation CPUs, so the CPU would still undoubtedly limit your performance in newer games.
And in general, most PCIe 2.x and 3.x graphics cards should be backward compatible with a PCIe 1.0 slot. Generally, it just limits the amount of throughput that the card has access to, though in practice, that shouldn't affect performance too badly.
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What if my psu is 300 watts can i still use gtx 650?or should i buy cheap graphic card like gt 730 or r7 240?
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560 or 650? I would actually not recommend a GTX 560 like the one that was linked to before, since it requires extra power connectors and can draw over 150 watts under load. Plus, the GTX 560 is quite old, and came out over 6 years ago. There are newer GPUs that perform the same or better while requiring less than half as much power. A 300 watt PSU can be cutting things close for graphics card upgrades, but many lower end cards get all their power from the PCIe slot, and won't draw more than around 70 watts under load. A GTX 650 is also pretty old, but it requires less power, though it also performs worse. I'd recommend going with something newer, at least a GTX 750, which should perform slightly better than a 560 while only drawing around 60 watts while gaming.
[quotemsg=19411284,0,2434774]do i need to add another 2GB ram?[/quotemsg]
I'd say yes. I believe your motherboard only has 2 RAM slots though, and can use up to 2GB in each slot. If one slot was empty, and the other had a single 2GB stick, you might be able to just add another single 2GB stick, though they would need to match specifications exactly to reduce the chances of running into issues. It would probably be best to use a matched set of 2x2GB sticks to avoid potential issues, and if your system currently has 2x1GB sticks, you would have to do this anyway. Buying a brand new set of RAM like Elbert linked to would likely work fine, though you could likely find a much better bargain on used DDR2 from someplace like eBay at this point. I was seeing some used 2x2GB sets of PC2-6400 DDR2 memory that would likely be compatible for under $20 shipped. You just have to make sure it's compatible, and not server or laptop memory or something. For upgrading older systems, the used market can often make sense, since plenty of people are selling off their older components.
[quotemsg=19411442,0,2434774]my windows version is 7 ultimate 64bit and do i need to buy a new HDD maybe 320 or 500GB? do i need to add or replace my existing HDD?
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Some new games require a lot of hard drive space, so adding a hard drive might be necessary, unless your current one is almost empty. If you're trying to get GTA5 to run, it requires more space than most games, at somewhere over 60 GB, and if you were installing it from someplace like Steam, it might require nearly double that for the initial download and installation process. As long as your computer case has room for another drive though, you can likely add it in addition to your existing drive, so that you don't have to reinstall your operating system or anything. When installing games, you'll just have to select the second drive as the place where you want to install them to.